An important analytical technique known as Nuclear Magnetic Resonance Spectroscopy (NMR) has become increasingly important since its invention in 1946. When atomic nuclei are placed in a constant, homogeneous static magnetic field of high intensity and simultaneously subjected to a certain specified radio frequency alternating field, a transfer of energy can take place in which the oriented nuclei are pictured as flipping to another orientation. If the RF energy is applied in pulses, when the pulses are off, the nuclei are said to relax and during this relaxation they process about the direction of the fixed magnetic field. If a coil of wire is positioned closely to the precessing nuclei of the sample, an electrical signal will be induced in the coil. This observed signal caused by the relaxing nuclei is known as the NMR signal. Since each nuclei, i.e., material, has a unique NMR signal, this spectrometer is important in a great many fields of research.
The apparatus of an NMR spectrometer is simple in principle, but in practice the design and manufacture are extremely demanding because of the very small signal generated by the precessing nuclei. In NMR, for example, any source of stray RF energy can introduce noise, especially if it excites nuclear resonances of the same nuclei that are not in the sample. For example, in proton NMR, since there are strong fixed magnetic fields and RF excitation fields in close proximity to the signal receiver coil, it is very important to reduce sources of proton resonance interference outside the sample of interest.
The part of the NMR spectrometer which contains the coils that interact with the sample is called the probe. The probe includes a saddle coil for providing the RF excitation to the sample and for detecting the precessing nuclei. This coil must be capable of being very accurately tuned and matched for maximum power transfer. Additionally, another coil is occasionally employed in a probe for irradiating the sample at high power. This coil is known as a decoupler coil, which is used in special experiments where broadband RF power is employed to disrupt coupling between different nuclei. In modern spectrometers it is also known to shape the excitation pulse in order to achieve selective excitations at higher power at selective excitation frequency for experiments. For tuning the coils, it is necessary to include in the probe fairly large capacitive reactance in the form of adjustable high voltage capacitors in order to tune to the frequency of interest, and to match the impedance of the probe to the transmission lines to the transmitter and receiver circuits for maximum power transfer. Arcing of these capacitor having teflon-air dielectric is a common problem. The probe is generally tuned for highest signal to noise ratio at the reciever.
It is an object of this invention to enable reliable high power tuned circuits for excitation of an NMR sample without introducing into the receiver spectral artifact due to the non-sample signal into the receiver that overlaps with the observed frequencies of the nuclei of the sample being tested.
It is a feature of this invention that the probe's high power capacitors are made from materials that do not contain hydrogen to eliminate interference from proton resonances of hydrogen.
It is a further feature of this invention that the high power probe capacitors employ fluorinated dielectric oil or grease, for example KRYTOX.RTM., a trademarked product of DuPont.